import: use dirstateguard instead of dirstate.invalidate

Before this patch, "commands.import()" uses "dirstate.invalidate()" as
a kind of "restore .hg/dirstate to the original status" during a failure.

But it just discards changes in memory, and doesn't actually restore
".hg/dirstate". Then, it can't work as expected, if "dirstate.write()"
is executed while processing.

This patch uses "dirstateguard" instead of "dirstate.invalidate()" to
restore ".hg/dirstate" at failure even if "dirstate.write()" is
executed before failure.

This patch also removes "beginparentchage()" and "endparentchange()",
because "dirstateguard" makes them useless, too.

This is a part of preparations to fix the issue that the recent (in
memory) dirstate isn't visible to external process (e.g. "precommit"
hook).

mercurial/commands.py

@@ -4213,7 +4213,7 b' def import_(ui, repo, patch1=None, *patc'
         cmdutil.bailifchanged(repo)
 
     base = opts["base"]
-    wlock = lock = tr = None
+    wlock = dsguard = lock = tr = None
     msgs = []
     ret = 0
 
@@ -4221,7 +4221,7 b' def import_(ui, repo, patch1=None, *patc'
     try:
         try:
             wlock = repo.wlock()
-            repo.dirstate.beginparentchange()
+            dsguard = cmdutil.dirstateguard(repo, 'import')
             if not opts.get('no_commit'):
                 lock = repo.lock()
                 tr = repo.transaction('import')
@@ -4262,18 +4262,16 b' def import_(ui, repo, patch1=None, *patc'
                 tr.close()
             if msgs:
                 repo.savecommitmessage('\n* * *\n'.join(msgs))
-            repo.dirstate.endparentchange()
+            dsguard.close()
             return ret
-        except: # re-raises
-            # wlock.release() indirectly calls dirstate.write(): since
-            # we're crashing, we do not want to change the working dir
-            # parent after all, so make sure it writes nothing
-            repo.dirstate.invalidate()
-            raise
+        finally:
+            # TODO: get rid of this meaningless try/finally enclosing.
+            # this is kept only to reduce changes in a patch.
+            pass
     finally:
         if tr:
             tr.release()
-        release(lock, wlock)
+        release(lock, dsguard, wlock)
 
 @command('incoming|in',
     [('f', 'force', None,